The invention relates to methods for producing chlorine dioxide, and to substances used in performing such methods. Each method produces chlorine dioxide by activating zeolite crystals (previously impregnated with a mixture of sodium chlorite and a water-retaining substance such as calcium chloride) with protons (from an acid or other proton generating species), or by activating an aqueous solution of a water-retaining substance (such as calcium chloride) and sodium chlorite with protons (from an acid or other proton generating species).
Zeolites are hydrated metal aluminosilicate compounds with well-defined (tetrahedral) crystalline structures. Because zeolite crystals (both natural and synthetic) have a porous structure with connected channels extending through them, they have been employed as molecular sieves for selectively absorbing molecules on the basis of size, shape, and polarity.
Volumes packed with zeolite crystals (for example, small zeolite crystals chosen to have size in the range from 0.2 mm to one quarter inch) have been employed in air (or other gas) and water filtration systems to selectively absorb contaminants from a flowing stream of water or gas.
U.S. Pat. No. 5,567,405, issued Oct. 22, 1996 (based on U.S. application Ser. No. 08/445,025, filed May 19, 1995), and U.S. Pat. No. 5,573,743, issued Nov. 12, 1996 (based on U.S. application Ser. No. 08/445,076), teach methods for producing zeolite crystals impregnated with one or more of sodium chlorite, acetic acid, phosphoric acid, and citric acid, and methods for producing chlorine dioxide by moving a fluid (e.g., air or water) relative to a bed of zeolite crystals impregnated with sodium chlorite, and moving the fluid relative to another bed of zeolite crystals impregnated with one of the following: phosphoric acid, acetic acid and citric acid. The two beds can be physically mixed together, or the fluid can flow sequentially through distinct first and second beds. These references also teach a method for filtering a fluid by producing chlorine dioxide in the fluid (in the manner described in this paragraph) and then absorbing the chlorine dioxide from the fluid.
U.S. patent application Ser. No. 08/704,086, filed Aug. 28, 1996, teaches a variation on the chlorine dioxide production method of U.S. Pat. No. 5,567,405, which includes the steps of moving a fluid through a first bed of impregnated zeolite crystals (impregnated with at least one of phosphoric acid, acetic acid, and citric acid) and then moving the fluid through a second bed of impregnated zeolite crystals (impregnated with sodium chlorite).
Chlorine dioxide (ClO2) is useful for killing biological contaminants (such as microorganisms, mold, fungi, yeast and bacteria) and for oxidizing volatile organic chemicals which can contaminate fluid.
It is known to produce chlorine dioxide by: activating a metal chlorite solution by adding an acid thereto, activating a powdered composition (or other dry composition) by adding water thereto, or preparing an activated dry composition which releases chlorine dioxide over time. U.S. Pat. No. 4,547,381 (issued Oct. 15, 1985) and U.S. Pat. No. 4,689,169 (issued Aug. 25, 1987) mention these three techniques for producing chlorine dioxide, and disclose in some detail one type of such an activated dry composition. They teach that this activated dry composition is a mixture of a xe2x80x9cdry inert diluent,xe2x80x9d a metal chlorite, and a dry agent capable of reacting with the metal chlorite in a dry state to produce chlorine dioxide. The metal chlorite can be sodium chlorite, and the dry agent can be a xe2x80x9cdry acidxe2x80x9d such as granular citric acid. The inert diluent can be diatomaceous earth, sodium chloride, sodium silicate, disodium sulfate, or magnesium chloride, or a combination of two or more thereof. The mixture releases chlorine dioxide over time until the rate of chlorine dioxide release becomes low, and the patents teach that the mixture can then be agitated for xe2x80x9crenewed generationxe2x80x9d of chlorine dioxide.
However, an activated composition (such as that described in U.S. Pat. Nos. 4,547,381 and 4,689,169) is subject to undesirable storage and shipping losses, due to outgassing of chlorine dioxide before the time of intended use of the composition.
Similarly, the inventor has found that sodium chlorite-impregnated zeolite crystals (of the type described in referenced U.S. Pat. Nos. 5,567,405 and 5,573,743) are also subject to undesirable storage and shipping losses, due to outgassing of chlorine dioxide therefrom before the time of their intended use (e.g., before fluid is caused to flow through both a bed of the sodium chlorite-impregnated zeolite crystals and a bed of acid-impregnated zeolite crystals). Also, efforts to activate sodium chlorite-impregnated zeolite crystals (of the type described in referenced U.S. Pat. Nos. 5,567,405 and 5,573,743) with acid at low temperature (below 40 degrees Fahrenheit) may fail in the sense that they will not result in release of sufficient amounts of chlorine dioxide.
There are also disadvantages to use of conventional metal chlorite solutions (of the type mentioned in U.S. Pat. Nos. 4,547,381 and 4,689,169) to produce chlorine dioxide. For example, when such a conventional solution is activated (by adding an acid thereto) to release chlorine dioxide gas, it is difficult or impossible to control the rate of release of the chlorine dioxide gas. Overproduction of chlorine dioxide often results.
There are also disadvantages to use of conventional powdered or dry compositions of the type activated by adding water thereto to release chlorine dioxide (as mentioned, for example, in U.S. Pat. Nos. 4,547,381 and 4,689,169). Masschelein, in the book Chlorine Dioxidexe2x80x94Chemistry and Environmental Impact of Oxychlorine Compounds (published 1979 by Ann Arbor Science Publishers Inc., Ann Arbor, Mich.) at page 140, describes one such dry mixture comprising sodium chlorite, and a solid organic anhydride containing 2 to 20% of a desiccating product such as calcium chloride. When such a conventional dry composition is activated (by adding water thereto) to release chlorine dioxide gas, it is typically difficult or impossible to control the rate of release of the chlorine dioxide to achieve chlorine dioxide release rates useful for such applications as air or water filtration. Overproduction of chlorine dioxide often results.
Until the present invention, it was not known how to produce a stable, unactivated substance (either in liquid or dry form) which does not release significant amounts of chlorine dioxide until activated by exposure to an acid, and which releases chlorine dioxide at a useful (and controllable) rate when exposed to an acid (even at temperatures below 40 degrees Fahrenheit). Nor had it been known to use such a stable, unactivated substance to produce controlled release of chlorine dioxide for filtering air or water.
In some embodiments, the invention is a method for producing chlorine dioxide by activating zeolite crystals (previously impregnated with sodium chlorite and calcium chloride) with excess protons or activating an aqueous solution of sodium chlorite and calcium chloride with excess protons. Typically, the excess protons are produced by exposing the crystals to an acid (or other proton generating species), or exposing the aqueous solution of sodium chlorite and calcium chloride to an acid (or other proton generating species). Examples of proton generating species useful for such activation are acids such as acetic acid, phosphoric acid, citric acid, HCl, propionic acid, and sulfuric acid, and metal salts such as ferric chloride, ferric sulfate, ZnSO4, ZnCl2, CoSO4, CoCl2, MnSO4, MnCl2, CuSO4, CuCl2, and MgSO4. In some embodiments the proton generating species releases excess protons upon exposure to the impregnated zeolite crystals themselves. In other embodiments the proton generating species must itself be activated to cause it release excess protons, so that the protons can in turn activate the impregnated zeolite crystals.
In accordance with the invention, the step of activating zeolite crystals with excess protons can be performed by causing a fluid (e.g., air, another oxygen-containing fluid, or water) to flow through a bed of zeolite crystals impregnated with sodium chlorite and calcium chloride, and a bed of zeolite crystals impregnated with the proton generating species. The two beds can be physically mixed together, or the fluid can be caused to flow sequentially through distinct first and second beds (preferably first through the bed containing proton generating species-impregnated zeolite, but alternatively first through the bed containing sodium chlorite and calcium chloride-impregnated zeolite). Alternatively, chlorine dioxide is produced using sodium chlorite and calcium chloride-impregnated zeolite crystals by immersing the impregnated zeolite in (or spraying the impregnated zeolite with) acid or another proton generating species, or otherwise exposing the impregnated zeolite to acid (preferably aqueous acetic, phosphoric, citric acid, HCl, sulfuric acid, propionic acid, or another acid, with a concentration of 0.025% to 0.5% in the sense that the acid comprises 0.025% to 0.5% by weight of the combined impregnated zeolite and acid) or another proton generating species.
To produce chlorine dioxide using the inventive aqueous solution of sodium chlorite and calcium chloride, the solution can be mixed (or otherwise combined) with acid. The liquid mixture can then be sprayed or coated on a surface (or the liquid mixture can be absorbed in a material such as a sponge, pad, mat, or the like, or simply placed in a reservoir, container, box, or the like) from which chlorine dioxide gas can escape at a desired rate.
In any of the embodiments, the rate of chlorine dioxide release (following activation) can be controlled in any of several ways, including by appropriately selecting the concentration and amount of the activating acid (or other proton generating species), and using impregnated zeolite having appropriately selected weight ratios of metal chlorite (e.g., sodium chlorite) or sodium chlorate to zeolite, and deliquescent or water absorbing and retaining substance (e.g., calcium chloride) to zeolite.
Other embodiments of the invention are substances useful for producing chlorine dioxide, which are stable until activated with excess protons (in the sense that they do not release chlorine dioxide gas in significant amounts until so activated with protons). One such embodiment is one or more zeolite crystals impregnated with sodium chlorite and calcium chloride. Preferably, the zeolite crystals are small (each of size in a range from 0.2 mm to one quarter inch). Also preferably, the crystals comprise 1%-6% sodium chlorite, 0.5%-2% calcium chloride, 0%-20% (4%-8% in some preferred embodiments) water, and 72%-98.5% (8%-94.5% in some preferred embodiments) zeolite by weight. Another such embodiment is an aqueous solution of sodium chlorite and at least one chemical selected from the group consisting of magnesium sulfate, potassium chloride, potassium hydroxide, and calcium chloride, preferably comprising 1%-6% of sodium chlorite, 0.5%-2% of said at least one chemical, and 92%-98.5% of water (by weight). Preferably, the solution comprises 1%-6% sodium chlorite, 0.5%-2% calcium chloride, and 92%-98.5% water (by weight).
In variations on any embodiment of the invention, a water-retaining substance other than calcium chloride, such as magnesium sulfate (MgSO4), potassium chloride, or potassium hydroxide, is substituted for calcium chloride. For example, zeolite crystals impregnated with sodium chlorite and magnesium sulfate are within the scope of the invention. For another example, a method of producing chlorine dioxide by activating an aqueous solution of sodium chlorite and magnesium sulfate with an acid is also within the scope of the invention.
In other variations on any embodiment of the invention, a metal chlorite other than sodium chlorite is substituted for sodium chlorite. For example, zeolite crystals impregnated with calcium chloride (or magnesium sulfate) and a metal chlorite other than sodium chlorite are within the scope of the invention.
The inventor has found that the presence (in a sufficient amount) of calcium chloride (or another water-retaining substance such as magnesium sulfate, potassium chloride, or potassium hydroxide) in the unactivated composition of the invention reduces the rate of chlorine dioxide outgassing to no more than a negligible amount at times prior to activation of the composition with protons, and yet allows release of chlorine dioxide at a desired rate following activation of the composition with protons. One of the reasons for chlorine dioxide outgassing (prior to activation with excess protons) from zeolite impregnated with metal chlorite (but not impregnated with a water-retaining substance such as calcium chloride) is migration of protons in the aluminosilicates comprising the zeolite.
In other embodiments, the invention is a method for producing chlorine dioxide, including the steps of providing a zeolite crystal mixture, wherein the mixture comprises zeolite crystals impregnated with sodium chlorate and zeolite crystals impregnated with an oxidizer, and activating the mixture with excess protons, and a zeolite crystal mixture for use in performing such method. In other embodiments, the invention is a method for producing chlorine dioxide, including the steps of providing a zeolite crystal mixture, wherein the mixture comprises zeolite crystals impregnated with a proton generating species and zeolite crystals impregnated with an oxidizer (or sodium chlorate), and causing the zeolite crystal mixture to come in contact with sodium chlorate solution (or an oxidizer, where the mixture includes zeolite crystals impregnated with sodium chlorate but not an oxidizer), and a zeolite crystal mixture for use in performing such method.